薄膜材料科学-第一章

MaterialsScienceofThinFilmsChapter1AReviewofMaterialsScience1.1INTRODUCTION•Solidsubstances:revealsoutwardlyappearstobeanendlessmultitudeofexternalformsandstructurespossessingabewilderingvarietyofproperties.•MaterialsScienceevolvedinparttoclassifythosefeaturesthatarecommonamongthestructureandpropertiesofdifferentmaterials.Categoriesofsolids-electronicstructure•SolidscanbeclassifiedasbelongingtypicallytooneofonlyfourdifferentcategoriesmetallicioniccovalentvanderWaals)•Dependingonthenatureoftheelectronicstructureandresultinginter-atomicbondingforces.Categoriesofsolids-engineering•Anotherschemebasedonengineeringusewouldagainarguablylimitmaterialstofourchiefclasses,namely,metalssemiconductorspolymersceramics.Categoriesofsolids-Crystalline•Solidsareeitherinternallycrystallineornoncrystalline.•Thosethatarecrystallinecanbefurthersubdividedaccordingtooneof14differentgeometricarraysorlatticesdependingontheplacementoftheatoms.1.2STRUCTURE1.2.1CRYSTALLINESOLIDS•Actualcrystalstructurescanbeimaginedtoarisefromathree-dimensionalarrayofpointsgeometricallyandrepetitivelydistributedinspacesuchthateachpointhasidenticalsurroundings.•ThereareonlyfourteenwaystoarrangepointsinspacehavingthispropertyandtheresultingpointarraysareknownasBravaislattices.Positions,Planes&DirectionsPositions:(u,v,w)Planes:(l,m,n)Directions:l,m,nSpecificcrystalplanesanddirections•Specificcrystalplanesanddirectionsarefrequentlynoteworthybecausephenomenasuchascrystalgrowth,chemicalreactivity,defectincorporation,deformation,andassortedpropertiesarenotisotropicorthesameonallplanesandinalldirections.•Therefore,theimportantneedarisestobeabletoaccuratelyidentifyanddistinguishcrystallographicplanesanddirections.Identifyingagivenplane-Millerindices•Asimplerecipeforidentifyingagivenplaneinthecubicsystemis:•Determinetheinterceptsoftheplaneonthethreecrystalaxesinnumberofunitcelldimensions.•Takereciprocalsofthosenumbers.•Reducethesetosmallestintegersbyclearingfractions.Directions•Perpendiculartoaspecificplane.•Foraspecificplane(l,m,n),directionisrepresentedasl,m,n,or[l,m,n].Theangleabetweenanytwodirectionsorplanes可通过软件求得。Thespacingbetweensuccessive(hkl)planes•wherea0isthelatticeparameter.example•[1,1,1]-typedirectionstobe[-1,-1,1]1.2.3AMORPHOUSSOLIDS•Insomematerialsthepredictablelong-rangegeometricordercharacteristicofcrystallinesolidsbreakdown.•Thesearethenoncrystallineamorphousorglassysolids.•Silicaglass,inorganicoxidemixtures,andpolymers.1.3DEFECTSINSOLIDSvacancies,dislocations,andgrainboundariesstructuraldefects•Perfectcrystalissomewhatofanexaggeration.carefullygrownsiliconsinglecrystalshavevirtuallyperfectcrystallographicstructuresextendingovermacroscopicdimensions.•Inthincrystallinefilmsthepresenceofdefectsnotonlyservestodisruptthegeometricregularityofthelatticeonamicroscopiclevel,butsignificantlyinfluencesmanyfilmpropertiessuchaschemicalreactivity,electricalconduction,andmechanicalbehavior.•Thestructuraldefectsbrieflyconsideredinthissectionarevacancies,dislocations,andgrainboundaries.1.3.1VACANCIES•Themostelementaryofcrystallinedefectsarevacancies,andtheyarisewhenlatticesitesareunoccupiedbyatoms.•Thefraction/oftotalsitesthatwillbeunoccupiedasafunctionoftemperatureTispredictedtobeapproximately.•NotingthatkBistheBoltzmannconstantandEfistheformationenergy,whichistypically1eVperatomgivesf=10-5at1000K.Rolesofvacancies•Vacanciesplayanimportantroleinallprocessesrelatedtosolid-statediffusion,includingrecrystallization,graingrowth,sintering,andphasetransformations.•Insemiconductors,vacanciesareelectricallyneutralaswellaschargedandcanbeassociatedwithdopantatoms.Thisleadstoavarietyofnormalaswellasanomalousdiffusional-dopingeffects.1.3.2DISLOCATIONS•Thesearelinedefectsthatbearadefinitecrystallographicrelationshiptothelattice.•Thetwofundamentaltypesofdislocations—theedgeandthescrew.•Anedgedislocationcanbegeneratedbywedginganextrarowofatomsintoaperfectcrystallattice,•Ascrewdislocationrequirescuttingfollowedbyshearingoftheresultanthalveswithrespecttoeachother.Burgersvector•Thegeometryofacrystalcontainingadislocationissuchthatwhenattemptingasimpleclosedtraverseaboutitsaxisinthesurroundinglattice,thereisaclosurefailure.•Onefinallyarrivesatalatticesitedisplacedfromthestartingpositionbyalatticevector,theso-calledBurgersvectorb.•Thisvectorliesperpendiculartotheedgedislocationlineandparalleltothescrewdislocationline.Straincausedbydislocations•Crystallatticearenowdistortedsomewhatinthepresenceofdislocations.•Therefore,evenwithoutapplicationofexternalforcesonthecrystal,astateofinternalstrain(stress)existsaroundeachdislocation.•Furthermore,thestrains(stresses)differaroundedgeandscrewdislocationsbecausethelatticedistortionsdiffer.•Closetothedislocationaxisorcorethestressesarehighbuttheyfalloffwithdistance(r)accordingtoa1/rdependence.Vacancies&dislocations•Incontrasttovacancies,dislocationsarenotthermodynamicdefects.•Coupledwithahighformationenergy•duetothemanyatomsinvolved,thermodynamicswouldpredictadislocationcontentoflessthanonepercrystal.•Thus,whileitispossibletocreateasolidd